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Winter biogenic silica and diatom distributions in the Indian sector of the Southern Ocean

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doi: 10.1016/j.dsr.2020.103421
Authors:Weir, Ian; Fawcett, Sarah; Smith, Shantelle; Walker, David; Bornman, Thomas; Fietz, Susanne
Author Affiliations:Primary:
Stellenbosch University, Department of Earth Sciences, South Africa
University of Cape Town, Cape Town, South Africa
Cape Peninsula University of Technology, Cape Town, South Africa
South African Environmental Observation Network, Port Elizabeth, South Africa
Nelson Mandela University, Port Elizabeth, South Africa
Volume Title:Deep-Sea Research. Part I: Oceanographic Research Papers
Source:Deep-Sea Research. Part I: Oceanographic Research Papers, Vol.166. Publisher: Elsevier, Oxford, International. ISSN: 0967-0637
Publication Date:2020
Note:In English. 83 refs.; illus., incl. sketch map, sect., 4 tables
Summary:Spring and summer Southern Ocean phytoplankton communities have been well characterized, but winter communities are often overlooked. Diatoms are a major contributor to Southern Ocean particulate organic carbon (POC) production and export, and exert a strong control on Antarctic surface and Subantarctic thermocline nutrient concentrations, thus influencing the low-latitude nutrient supply. Understanding diatom distribution, seasonal community progression, and diatom-nutrient interactions is vital for improving biogeochemical models, particularly as climate change alters polar phytoplankton communities. We investigated the distribution of nanophytoplankton (≥3 µm) and their associated biogeochemical environments along 30°E across the Indian Southern Ocean (Subtropical Zone; STZ to Antarctic Zone; AZ) in July 2017. Phytoplankton productivity inferred from chlorophyll-a was low compared to previously-published summertime values. Mixed-layer nitrate (NO3-) and phosphate (PO43-) concentrations were similar to existing summer measurements, likely due to a combination of deep mixing and biological uptake, while silicic acid (Si(OH)4) was high relative to summer, particularly south of the Polar Front (PF). The PF emerged as an important biogeochemical boundary separating relatively high chlorophyll-a, flagellate-dominated northern waters from southern waters characterized by low chlorophyll-a and diatom-dominated communities. Mixed layer-integrated biogenic silica (bSi) decreased 12-fold from the southern AZ to the STZ, resulting in a strong south-north gradient in bSi-per-chl-a (from 3.6 to 0.1 mol/g) and bSi-per-POC (from 0.22 to 0.016 mol mol-1). We attribute this to a high abundance of heavily-silicified diatom species (e.g., Fragilariopsis spp., which dominated the AZ diatom community) and a limited contribution of other phytoplankton to chlorophyll-a and POC to the south - indeed, diatoms constituted 5-67% of the total POC south of the PF and only 3-7% to the north. While mixed-layer Si(OH)4 concentrations decreased more than NO3- across the PF, likely due to preferential Si(OH)4 consumption by iron-limited diatoms, our data imply a lower ratio of Si(OH)4 to NO3- uptake compared to summer. This suggests that iron limitation may be less severe in the AZ in winter, at least in the west Indian sector. We conclude that AZ diatoms impact the low-latitude nutrient supply and are potentially important for carbon export in winter, despite the lower productivity of the Southern Ocean during this season.
Subjects:Algae; Carbon; Diatoms; Equations; Microfossils; Nutrients; Organic carbon; Phytoplankton; Plankton; Seasonal variations; Silica; Statistical analysis; Antarctic Ocean; Fragilariopsis
Coordinates:S583000 S410000 E0350000 E0330000
Record ID:892033-11
Copyright Information:GeoRef, Copyright 2021 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands
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